chore: s.remove_staging_dirs() should only be called once (#57)

There is [an issue](https://github.com/googleapis/python-workflows/blob/master/owlbot.py#L46) in the `owlbot.py` file added in #50 in that [s.remove_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L309) should only be called once after all the files are copied over.  [get_staging_dirs()](https://github.com/googleapis/synthtool/blob/master/synthtool/transforms.py#L280) will only return staging directories that exist.

owlbot.py

@@ -43,8 +43,6 @@
             ],
         )
 
-s.remove_staging_dirs()
-
 # move workflows after executions, since we want to use "workflows" for the name
 for library in s.get_staging_dirs(workflows_default_version):
     if library.parent.absolute() == 'workflows':
@@ -69,4 +67,4 @@
 )  # the microgenerator has a good coveragerc file
 
 
-s.shell.run(["nox", "-s", "blacken"], hide_output=False)
+s.shell.run(["nox", "-s", "blacken"], hide_output=False)